Fix people index for non-admin access (#1490)

[maebeale]

Closes #1490

What is the goal of this PR and why is this important?

• In preparation for exposing the Person profile to non-admins, harden the /people index so it doesn't crash on a search filter and renders only links the viewer can follow.

How did you approach the change?

• Ambiguous end_date: qualified the column inside Affiliation.active (affiliations.end_date) so merging it onto a relation that also joins organizations (which has its own end_date) no longer raises Trilogy::ProtocolError: 1052: Column 'end_date' in where clause is ambiguous. Added regression tests that join :organization and that chain Person.with_active_affiliations.search_by_params(organization_name: ...).
• Policy-aware links in app/views/people/people_results.html.erb: gated the person profile button, organization affiliation pills, and the overflow +N link with allowed_to?(:show?, ...). When the viewer lacks access, the same content renders as plain text instead of a link. Expanded the fragment-cache key to include whether the viewer owns the row so policy-gated output stays correct.
• Note: the locked-user filter and the index policy change are not in this PR — those landed in Hide locked users' profiles from the non-admin people index #1489 / HOLD UNTIL READY TO LAUNCH: Allow authenticated users to access the people index and search #1494.

UI Testing Checklist

• As an admin, /people renders with all existing links (person profile, organization pills, edit, user)
• As an admin, filtering by organization_name no longer 500s (regression check for the ambiguous column)
• (Once the index opens to non-admins) confirm organization pills render as plain text when OrganizationPolicy#show? returns false
• (Once the index opens to non-admins) confirm an owner sees a working link to their own profile row

Anything else to add?

• The index controller still uses PersonPolicy#index? (admin-only on main), so the view changes are dormant until the policy opens up. The third issue checklist item ("check policies for all links in the index results") is addressed defensively rather than waiting for the policy change.

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